Design Patterns II Department of Computer Science University of - - PowerPoint PPT Presentation

CMSC 132: Object-Oriented Programming II

Design Patterns II

Department of Computer Science University of Maryland, College Park

Observer Pattern

• Definition

– Updates all dependents of object automatically once

• bject changes state
• Where to use & benefits

– One change affects one or many objects – Many object behavior depends on one object state – Need broadcast communication – Maintain consistency between objects – Observers do not need to constantly check for

changes

Observer Pattern

• Example

–

Multiple windows (views) for single document

–

Original

• Each window checks document
• Window updates image if document

changes

• Think of window as asking “Are we there

yet?”

–

Using pattern

• Each window registers as observer for

document

• Document notifies all of its observers

when it changes

Doc Window Doc Window Window Window

Observer Example

public interface Observer { // called when observed object o changes public void update(Observable o, Object a) } public class Observable { protected void setChanged() protected void clearChanged() boolean hasChanged() void addObserver(Observer o) void notifyObservers() void notifyObservers(Object a) }

public class MyWindow implements Observer { public openDoc(Observable doc) { doc.addObservers(this); // add window to list } public void update(Observable doc, Object arg) { redraw(doc); // display updated document } } public class MyDoc extends Observable { public void edit() { … // edit document setChanged(); // mark change notifyObservers(arg); // invokes update() } }

State Pattern

• Definition

–

Represent change in an object’s behavior using its member classes

• Where to use & benefits

–

Control states without many if-else statements

–

Represent states using classes

–

Every state has to act in a similar manner

–

Simplify and clarify the program

• Example

–

States representing finite state machine (FSM)

–

Original

• Each method chooses action depending on state
• Behavior may be confusing, state is implicit

–

Using pattern

• State interface defines list of actions for state
• Define inner classes implementing State interface
• Finite state machine instantiates each state and tracks its current state
• Current state used to choose action
• Example: StateCode

State Example – Original Code

public class FickleFruitVendor { boolean wearingHat; boolean isHatOn() { return wearingHat; } String requestFruit() { if (wearingHat) { wearingHat = false; return "Banana"; } else { wearingHat = true; return "Apple"; } } } Wearing Hat Not Wearing Hat Banana Apple

State Example

public interface State { boolean isHatOn(); String requestFruit(); } public class FickleFruitVendor { State wearingHat = new WearingHat(); State notWearingHat = new NotWearingHat(); // explicitly track current state of Vendor State currentState = wearingHat; // behavior of Vendor depends on current state public boolean isHatOn() { return currentState.isHatOn(); } public String requestFruit() { return currentState.requestFruit(); } }

public class WearingHat implements State {

boolean isHatOn() { return true; } String requestFruit() { // change state currentState = notWearingHat; return "Banana"; } } public class NotWearingHat implements State { boolean isHatOn() { return false; } String requestFruit() { // change state currentState = wearingHat; return "Apple"; } } }

Wearing Hat Not Wearing Hat Banana Apple

Visitor Pattern

• Definition

–

Define operations on elements of data structures without changing their classes

• Where to use & benefits

–

Add operations to classes with different interfaces

–

Can modify operations on data structure easily

–

Encapsulate operations on elements of data structure

–

Decouples classes for data structure and algorithms

• Example

–

Print elements in collection of objects

–

Original

• Iterator chooses action based on type of object
• Many if-else statements

–

Using pattern

• Visitor interface defines actions during visit
• Visitable interface allow objects to accept visit
• Action automatically selected by polymorphic functions through double dispatch
• Example: VisitorCode

Visitor Example – Original Code

public void messyPrintCollection(Collection c) { for (Object o : c) { if (o instanceof String) System.out.println(“{"+o.toString()+“}"); // add { } else if (o instanceof Float) System.out.println(o.toString()+"f"); // add f else System.out.println(o.toString()); } }

Visitor Example

public class VisitableString implements Visitable { private String value; public VisitableString(String s) { value = s; } public String toString( ) { return value.toString( ); } public void accept(Visitor v) { v.visit(this); } } public class VisitableFloat implements Visitable { private Float value; public VisitableFloat(Float f) { value = f; } public String toString( ) { return value.toString( ); } public void accept(Visitor v) { v.visit(this); } } public interface Visitor { public void visit(VisitableString s); public void visit(VisitableFloat f); } public interface Visitable{ public void accept(Visitor v); } public class PrintVisitor implements Visitor { public void visitCollection(Collection c) { for (Object o : c) { if (o instanceof Visitable) ((Visitable) o).accept(this); else System.out.println(o.toString()); } } public void visit(VisitableString s) { System.out.println(“{"+s.toString( )+“}"); } public void visit (VisitableFloat f) { System.out.println(f.toString( )+"f"); } }